1. Field of the Invention
The present invention relates to a method for producing an electrolytic solution for redox batteries and, more particularly, to a method for simultaneously producing both trivalent and tetravalent vanadium electrolytic solutions.
2. Description of the Related Art
In recent years, through global problems such as acid rain, the destruction of the ozone layer by fluorocarbons, and the greenhouse effect due to an increase in carbon dioxide in the atmosphere, environmental problems are being focussed upon as a problem for all mankind. In the midst of this state of affairs, the movement to make the fullest possible use of solar energy, an inexhaustible and clean form of energy that is friendly towards the earth, has increased considerably. Examples of this include solar batteries, power generation making use of solar heat or heat reclamation, wind-turbine generation, and wave power generation (power generation making use of the energy of ocean currents, or temperature differences of seawater).
Of all of these, it is solar batteries, with a remarkable revolution in technology, that show signs of heading towards the time when they are ready for genuine practical applications, through improvements in their efficiency and a significant lowering in price. Currently, use of solar batteries is restricted to rather small-scale applications such as in powering road signs and communications relays, but rapid developments are also expected through envisioned solar energy cities and the implementation of designs to lay fields of batteries in the oceans or deserts. However, the power output of all of these power generation methods making use of solar energy is affected by climactic conditions, thus making stable and trustworthy production of electrical power impossible. Coordinated use of solar energy and reliable, effective batteries are required, and the realization there of has been long awaited.
Moreover, electrical power may be easily converted into other types of energy, is easy to control, and causes no environmental pollution at the time of its consumption, and it is for these reasons that the percentage of total consumption taken up by electrical power is increasing every year. The distinguishing characteristic of electrical power is that its production and consumption are simultaneous with each other, and that it cannot be stored. It is for this reason that, at present, highly efficient nuclear power generation and advanced thermal power generation are being operated at the highest possible efficiency ratings, and that increase in the large demand for electricity during daylight hours is being met by with small-scale thermal and hydropower generation suitable for generating power in response to fluctuations in consumption of electrical power; thus the current state of affairs is such that excess energy is being produced at night. The power generation world is earnestly hoping for development of technology that will make it possible to store this excess energy at night and use it efficiently during the day.
From circumstances such as those above, all types of secondary batteries have been studied as a method of storing electrical energy which does not pollute the environment and as an energy with a wide variety of applications. Redox batteries have received special attention as a high-volume stationary battery capable of operating at room temperatures and atmospheric pressure.
Redox batteries pass an electrically active materials of postive and negative solution to the cells with flow-through electrodes, and making use of a redox reaction, perform charging and discharging of batteries, and thus have a comparatively longer life than normal secondary batteries, with minimized self-discharging, and possess the advantages of being high in both reliability and safety. In recent years the actualization of redox batteries have received considerable attention.
At present, redox batteries which are held to be in the stage of practical use, that is those with redox couple of bivalent and trivalent chromium vs. bivalent and trivalent iron, cannot be made to have concentrated solutions due to crossmixing with iron and chromium passing through the membrane of the cells and limitations on solubility. Also, with an output voltage for a single cell of approximately 0.9-1 volts (V), their energy density is low. Furthermore, when the charged state of the electrodes becomes unequal due to generation of hydrogen on the negative electrode, there is the danger of generating chlorine from the positive electrode during charging.
On the other hand, there has also been a proposal for a redox battery with postive and negative electrodes which have trivalent and bivalent ion pairs, and tetravalent and pentavalent vanadium dissolved in a sulfuric solution (U.S. Pat. No. 4,786,567, Journal of Power Sources 15 (1985) 179-190 and 16 (1985) 85-95). This battery has a high output voltage of 1.4V to 1.5V, and is characterized by its high efficiency and high energy density, but in order to obtain a high-density vanadium solution, costly vanadyl sulfate must be used, and this has been viewed as being poorly suited for practical use.
In order to solve the above problems, the present inventors have proposed in U.S. Ser. No. 07772,794 now U.S. Pat. No. 5,250,158 a method for producing at low cost a vanadium electrolytic solution by reducing, in the presence of an inorganic acid, vanadium compounds recovered from the ash produced by combustion of heavy oil fuels.
A redox battery comprises end plates; positive and negative carbon cloth electrodes provided between the end plates; and a separating membrane which is made of an ion exchange membrane and placed between the positive and negative carbon cloth electrodes. The electrolytic solutions for the positive and negative electrodes are supplied to the respective electrodes from respective tanks. To initially charge the redox battery, a tetravalent vanadium electrolytic solution is put into both of the tanks and, then, electrolytic reduction is performed. Tetravalent vanadiums are oxidized to become pentavalent vanadiums at the positive electrode and reduced to become trivalent vanadiums at the negative electrode. Then, the resultant pentavalent vanadium electrolytic solution at the positive electrode must be replaced with or made into a tetravalent vanadium electrolytic solution before the redox battery can be used. This solution replacement requires substantial work and specialized equipment, thus increasing production costs. Because of this problem, there is a need for a method in which a trivalent vanadium electrolytic solution and a tetravalent vanadium electrolytic solution are simultaneously prepared. Preferably, a chemical reduction method should be employed instead of the electrolytic reduction to prepare a trivalent vanadium electrolytic solution, because chemical reduction is more economical.
Methods for chemically reducing pentavalent vanadium compounds to trivalent vanadium compounds are described in "Inorganic Chemistry" (Maruzen Asian Edition) by R. B. Heslop and P. L. Robinson. These methods are conducted as follows: EQU V.sub.2 O.sub.5 .fwdarw.H.sub.2 (heated).fwdarw.V.sub.2 O.sub.3 +2H.sub.2 O EQU V.sub.2 O.sub.5 .fwdarw.CS.sub.2 (heated).fwdarw.V.sub.2 S.sub.3 EQU V.sub.2 O.sub.5 .fwdarw.Zn (in sulfuric acid).fwdarw.V.sub.2 (SO.sub.4).sub.3
These methods are not suitable for preparing a vanadium electrolytic solution for a redox battery, because the trivalent vanadium compounds produced by these methods are mixtures or contaminated with metals which are not desired in a vanadium electrolytic solution.
"Inorganic Synthesis", vol. 7, p. 92, describes a method which heats a concentrated sulfuric acid solution containing vanadium pentaoxide and sulfur. However, the reaction rate in this method is very slow. Further, the resultant vanadium sulfate V.sub.2 (SO.sub.4).sub.3 must be separated from unreacted sulfur by suspending the resultant mixture in a carbon disulfide-50% ethanol mixed solution, decanting sulfur aggregated on the suspension surface, filtering the remaining suspension, and rinsing the thus-obtained solid with water, and thus obtain the vanadium sulfate V.sub.2 (SO.sub.4).sub.3. This process must be repeated many times to substantially remove unreacted sulfur from the vanadium sulfate. Further, because vanadium sulfate is insoluble in diluted and concentrated sulfuric acid solutions, it cannot be immediately used to prepare a trivalent vanadium electrolytic solution. Thus, this method is not suitable for the industrial-scale production of a trivalent vanadium electrolytic solution.